The present invention relates generally to chemical vapor deposition processes, and more specifically, to a chemical vapor deposition method for amorphous silicon and the resulting film.
A plasma-enhanced chemical vapor deposition (PECVD) process is a process widely used in the manufacture of semiconductor devices for depositing layers of electronic materials on various substrates. In a PECVD process, a substrate is placed in a vacuum deposition chamber equipped with a pair of parallel plate electrodes or other means of coupling electrical energy into the chamber, such as a helical coil. The substrate is generally mounted on a susceptor which is also the lower electrode. A flow of a reactant gas is provided in the deposition chamber through a gas inlet manifold which also serves as the upper electrode. A radio frequency (RF) voltage is applied between the two electrodes which generates RF power sufficient to cause the reactant gas to form a plasma. The plasma causes the reactant gas to vigorously react and deposit a layer of the desired material on the surface of the substrate body. Additional layers of other electronic materials can be deposited on the first layer by providing in the deposition chamber a flow of a reactant gas containing the material of the additional layer to be deposited. Each reactant gas is subjected to a plasma which results in the deposition of a layer of the desired material.
The plasma-enhanced chemical vapor deposition (PECVD) process can be utilized to deposit an amorphous silicon film that can be used in a variety of different applications. One such application is the manufacture of amorphous silicon-based circuit elements where the deposition of a-Si:H film on the silicon wafer is first necessary.
Unfortunately, it is difficult to produce good quality hydrogenated amorphous silicon (a-Si:H) films at high deposition rates to achieve manufacturing efficiency and high throughput. For example, in order to produce good quality films, a typical deposition rate used by conventional processes in the industry is approximately 1 to 10 Angstroms/s. At such a low deposition rate, the substrate manufacturing process is very inefficient. For example, it may take two to three hours just to deposit a one micron thick a-Si:H film at these low deposition rates. As can be appreciated, these slow process times are unacceptable for high volume integrated circuit manufacturing. However, when the deposition rate is increased, conventional processes produce films of unacceptable quality.
There have been some attempts to produce high quality amorphous silicon and to improve growth rates. One such study was reported by A. Matsuda in the J. Vac. Sci. Technol. A., Vol. 16, No. 1, January/February 1998 in an article entitled, xe2x80x9cPlasma and Surface Reactions for Obtaining Low Defect Density Amorphous Silicon at High Growth Rates.xe2x80x9d This publication recites from a plasma chemistry perspective some of the basic mechanisms that create a-Si:H films and provides some general comments regarding maintaining an xe2x80x9cadequate flow rate of silanexe2x80x9d and xe2x80x9creducing the partial pressure of silane.xe2x80x9d Unfortunately, the publication neither recites nor suggests concrete experimental results as to the deposition rate and the quality of the resultant amorphous silicon film. In addition, there is no suggestion or teaching regarding the specific process conditions or process details that can be utilized in a manufacturing process. Consequently, it would be desirable for there to be specific details on how to improve the growth rate of amorphous silicon and yet produce high quality material in a manufacturable process.
Some prior art approaches attempt to improve the growth of amorphous silicon by modifying the power applied to create the plasma in the CVD chamber. For example, U.S. Pat. No. 5,648,293 is directed to a method of depositing amorphous silicon by utilizing an improved high frequency discontinuous discharge. Unfortunately, controlling the high frequency discontinuous discharge requires additional overhead and is more complex to monitor, thereby injecting inefficiencies into the manufacturing process. Furthermore, the discontinuous discharge can damage devices that are on the silicon substrate.
Some prior art approaches attempt to improve the quality of the films by designing specialized processing equipment (e.g., a re-design of the reactor and load-lock system). One such approach is described in the article, xe2x80x9cDeposition of Ultrapure Hydrogenated Amorphous Siliconxe2x80x9d by Toshihiro Kamei and Akihisa Matsuda. Unfortunately, not only are these approaches costly to design, but the complexity of the equipment can make the equipment more cumbersome to operate and maintain. As can be appreciated, requiring new specialized equipment that must be tested and validated for large quantity runs injects complexity and inefficiencies into the manufacturing process.
Based on the foregoing, it is clearly desirable to provide a method for efficiently depositing an amorphous silicon film.
It is an object of the present invention to provide a method of depositing a-Si:H layer or film on a silicon substrate at high deposition rates such that it can be suitably used in a manufacturing process.
It is another object of the present invention to provide a method of depositing a-Si:H layer or film on a silicon substrate at high deposition rates while producing films of superior quality suitable for use in a manufacturing process.
According to one embodiment, the present invention provides a method of depositing a layer of amorphous silicon film on a substrate at a very fast deposition rate while maintaining superior film quality. A plasma volume in a process chamber is defined. A total flow rate of a mixture of gases introduced into the chamber is also defined. The total flow rate is the sum of the flow rates of the respective gases in the mixture. Next, a process parameter that includes the plasma volume and total flow rate is defined. The process parameter is then maintained in a first predetermined relationship with a predetermined value during the deposition of the amorphous silicon film. By using the present invention, circuit devices (e.g., diodes or thin film transistors) of superior quality can be produced at very high efficiency in an active semiconductor layer of amorphous silicon. The processing method of the present invention can be applied whenever a layer (doped or non-doped) of amorphous silicon is needed.
According to another embodiment, the present invention is directed to a high quality amorphous silicon film that has elements of silicon and hydrogen, a smooth and glassy appearance when viewed at 50,000xc3x97 magnification on a scanning electron microscope (SEM), and, when cleaved, a void-free morphology in the cross sectional view and plan views. The high quality amorphous silicon film of the present invention also has a density of states versus the depth in energy from the conduction edge of the bandgap plot substantially as shown in FIG. 5 and a photocapacitance versus photon energy graph substantially as shown in FIG. 6.